LED (light emitting diode) light sources sometimes employ phosphor materials to alter the wavelength of the light emitted by the LED. For example, a blue LED may be used with a yellow phosphor. The yellow phosphor absorbs some of the primary blue light from the LED and re-emits it as secondary yellow light. The yellow light combines with the unabsorbed blue light to form white light. In this example some blue light is intentionally left unconverted. This is possible because the phosphor particles are large and some of the light passes through gaps between the particles.
In a further example, an ultraviolet (UV) LED produces UV radiation that is converted by red, green and blue phosphors to form white light. In this example, it is undesirable for any of the primary UV radiation to pass through the phosphor without wavelength conversion, since the UV radiation is harmful to humans. Thus, it is important that leakage of UV radiation is minimized.
One way to reduce UV radiation leakage is to increase the thickness of the wavelength converting material. However, such light sources are inefficient, since some of the secondary (red, green and blue) radiation is reabsorbed by the phosphor.
Another way to reduce UV radiation leakage is to use smaller phosphor particles so that they can be closely packed together to reduce gaps between particles. Again, the conversion efficiency is reduced because smaller phosphor particles have inherently poorer light conversion efficiency.